The present invention relates to a fixing device for a printer, facsimile apparatus, copier or similar electrophotographic image forming apparatus. More particularly, the present invention relates to a fixing device of the type generating heat based on electromagnetic induction.
Generally, an image forming apparatus of the type using toner for developing a latent image includes a fixing device implemented by a heat roller and a press roller. The heat roller and press roller fix a toner image formed on a paper sheet or similar recording medium with heat and pressure while conveying the recording medium. A halogen lamp has customarily been disposed in the heat roller as a heat source for heating the inner periphery of the heat roller to a preselected temperature. A problem with the halogen lamp is that a substantial period of time is necessary for the lamp to heat the heat roller to the preselected temperature. This, coupled with the substantial loss of the halogen lamp itself, makes the lamp undesirable from the global environment standpoint.
In light of the above, a fixing device using induction heating is attracting increasing attention as a fixing device that is efficient and needs a minimum of warm-up time. This type of fixing device includes an induction heating device implemented by an induction coil. The induction coil is disposed in a heat roller and wound around a cylindrical bobbin formed of an insulator. High frequency current is fed to the induction coil via leads extending out from opposite ends of the coil. The high frequency current forms a high frequency magnetic field which in turn generates induced eddy current in the core or metallic conductor of the heat roller. As a result, the heat roller itself generates heat due to its own skin resistance on the basis of Joule heat. In this manner, the surface of the heat roller is heated to a desired temperature.
In the above induction heating device, the induction coil is disposed in the heat roller in place of the conventional halogen lamp so as not to obstruct the conveyance of a paper sheet. The induction coil therefore heats the entire inner periphery of the heat roller like a halogen lamp. This brings about a problem that heat generated in portions other than the nip between the heat roller and a press roller, which needs thermal energy, is simply wasted.
Technologies relating to the present invention are disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 7-295414, 8-137311, 11-297463, and 11-311910.